The present invention relates to a rolling mill which is suitable for cold rolling of a thin strip required of high quality such as material for lead frames, shadow masks, etc. and, more particularly, to a drive apparatus for driving rolls of a rolling mill, a rolling mill having the rolling mill drive apparatus and a rolling method.
Rolling mills for rolling a thin plate required of a high quality such as material for lead frames, shadow masks, etc. are used widely in order to roll material to make the thickness thin. The types of rolling mills are a 2-high rolling mill, a 4-high rolling mill and a 6-high rolling mill which rapidly is being widely used in recent years. Further, there are cluster type 12-high or 20-high rolling mills represented by sendzimir mills. Further, usually, it is necessary to drive two rolls in order to supply a power necessary for rolling, it is of course for drive rolls to be work rolls in a 2-high rolling mill, and it is fundamental that drive rolls are work rolls even in a 4-high rolling mill or in a more-than-4-high rolling mill. However, in a case of rolling a hard and thin material, it is necessary to make the diameter of a work roll small, in this case, the driving system becomes weak, so that a backup roll in a case of a 4-high rolling mill or an intermediate roll in a case of a 6-high rolling mill is driven. In a case of a cluster rolling mill, since the work rolls each have a small diameter in general, intermediate rolls (for example, 4 rolls in a case of a 20-high rolling mill) are driven.
Each of those rolls, in general, is connected to a spindle and driven by one electric motor through a gear type pinion stand. A work roll drive system of a 4-high rolling mill, which is a most typical example, is explained, referring to FIG. 14. In FIG. 14, power of an electric motor 100 is transmitted to an upper pinion 102 of a gear type pinion stand 101a through a coupling shaft 101. The power drives an upper work roll 106 through an upper spindle 104. On the other hand, the upper pinion 102 transmits the power to a lower pinion 103, the power is transmitted to a lower work roll 107 through a lower spindle 105, thereby to execute rolling. The upper and lower work rolls 106, 107 are supported by upper and lower backup rolls 108, 109, respectively. Here, the gear type pinion stand 101a is an important machine serving the role of a distributor for distributing the power from one electric motor 100 to two rolls to drive them.
There is a twin drive system which drives individually two rolls by two electric motors, respectively, which system is different from the drive system driving 2 rolls by one electric motor as mentioned above. This is used in a case of a large-sized rolling mill which employs a backup roll drive system and in a case where a gear type pinion stand is prevented from becoming huge in size. Further, this system may be applied, in some cases, to a work roll drive system in order to attain an advantage that rolling can be effected without managing strictly a difference between work roll diameters even in the work roll drive system. In this case, however, pinion and gears are necessary to secure a space for two electric motors and direct connection between the electric motors and the work rolls is difficult without the pinion and gears.
On the other hand, a conventional roll drive system without using a gear type pinion stand and spindle is disclosed in JP A 55-77916. This system has a construction in which a rolling roll is driven by directly bringing a drive roller into contact with the rolling roll without using the gear type pinion stand and spindle.
The conventional roll drive system which uses the gear type pinion stand and spindle is as mentioned above, and the system has a large number of points to be improved. The description about the points is as follows.
(1) The quality of plate surface:
Material for electronics represented by lead frames for semiconductors, shadow masks, etc. is required to be thinner and to have a higher quality. As for the improvement of the quality, plate thickness and plate shape precision has reached a stage to satisfy the requirements of recent technical developments. However, there are still left cases wherein very fine marks occur on plate surfaces and detract from the quality. There are various causes therefor, and one of which is that a rolling roll drive mechanism has gears. That is, when the gears are used in the driving system of rolling rolls, a driven gear can not attain correct rotational speed because of errors in tooth shape and errors in pitch, and a slight change in speed occurs. This change results in a cause for creating the marks on the plate surfaces. One of other causes is that there is a backlash which always exists in the gears. In a case where a thin plate is rolled with small torque, the driven gears change in speed by increasing or decreasing in the rolling speed because of the backlash, whereby vibrations are induced and marks are generated on the plate surfaces. This is similar to a drive system of a coiler for winding up a plate after rolling. In the coiler, the cause disappears by employing a direct connection system for directly connecting the coiler and an electric motor without using gears. In the rolling roll drive mechanism, also, a direct connection system can be employed by employing a twin drive system in a backup drive system, however, two electric motors are required, and the electric motors each are required to be large in size and low in rotation well enough to be able to drive a large-sized backup roll. Further, two control systems are required for controlling them, and the cost becomes high.
(2) Spindle rupture accident:
Of spindles used mainly for cold rolling apparatus, there are a gear type apparatus and a cross pin type apparatus using a rolling bearing. Recently, the latter has been used more widely because the latter has a higher efficiency and excellent maintenance operation. However, since the cross pin type apparatus is less in strength than the gear type apparatus in a case where plate rupture occurs and an excessive load is applied, it has a weak point that the rolling is caused to come to a rest by occurrence of the rupture accident. Further, in a high speed tandem mill, an excessive torque occurs by squeezing a material at a time of plate rupture, etc., and a spindle of a weak portion, or in some cases such a large accident as a tooth portion of a pinion is broken occurs sometimes. Such an accident, even when a spindle having the same strength is used, occurs very rarely at a rolling mill of front stage in which rolling torque is large, and in most cases, it occurs at a rolling mill of final stage at which torque is smallest. This is concerned with the fact that plate rupture occurs most frequently at the stand at which plate thickness becomes thin. It is difficult to detect correctly an abnormal torque due to the rolling trouble, however, in usual the torque is estimated to reach 800% of the usual maximum torque of an electric motor from the rupture conditions of the driving portion. As a countermeasure for this, in general, it is considered to use a shear pin, however, the shear pin has a ratio between the fatigue limit and the final strength, of about 3:4 and the shear pin may not be a sufficient protection means. Additionally, much time is required for exchanging the shear pin and efficiency is not good.
(3) Roll damage in a case of backup roll or intermediate roll drive system:
In a case of a backup roll drive of a 4-high rolling mill for instance, an excessive load is rapidly applied on work rolls when a rolling trouble such as plate rupture, squeezing, etc. occurs, so that the work rolls can not be rotated with frictional force from the backup rolls, and the work rolls rapidly decelerate and stop. On the other hand, the backup roll directly connected to an electric motor requires a long time until it stops because the backup roll including the electric motor has large inertia, during that time even if a screw-down operation for the work rolls is released, the backup roll continues to rub the work rolls for a relatively long time. As a result, the work rolls are shaved out to be in a half-moon shape, and run into a fatal damage accident, so that in some cases a roll cost may be increased to several times one of a work roll drive system. Therefore, even if a work roll of small diameter is desirable in order to roll a hard and thin material, in some case, the work roll drive system in which the diameter of the work roll is made larger has to be taken in view of the above-mentioned. This is similar to in a case of a 6-high rolling mill.
(4) Difficulty to effect high speed rolling:
Although high speed rolling is necessary to improve the productivity, when plate rupture occurs during the high speed rolling, damages of devices thereof become large and repairing cost increases and further non-operating time increases. Even without such things, much time is required for treating cobbles. Therefore, in rolling of metal plate (very thin material) which is thin and required of high quality such as lead frame material, shadow mask material, etc., in many cases, rolling at a low speed has to be effected without effecting high speed rolling necessary to improve the productivity.
On the other hand, in a conventional roll drive system without using a gear type pinion stand and a spindle, which system is disclosed in JP A 55-77916, since a drive roller is directly contacted with a rolling roll, it is inevitable that a rolling oil adheres between the drive roller and the rolling roll, causing the friction coefficient between the drive roller and the rolling roll to be reduced to a small value and loss occurs in the load applied to the drive roller. Therefore, it is necessary to apply a load of an amount corresponding to a rolling load from the drive roller. The electric motor is required to have very large power, which results in a large-scaled facility which is high in cost. Further, in this case, once the rolling oil is adhered between the drive roller and the rolling roll, it is very difficult to remove it. For example, it is difficult to remove completely the oil without taking such a method as burning it out, and a lot of labor is required.